Re: how is system geared down?

["Alexei Pytel" <pytel@xxxxxxxxxx>]

I do not work in processor development, so I do not know if this is true.
One question though - why officially announce processor point at 200Mhz if
in reality it works at say 100Mhz.
Is it even legal ?

    Alexei Pytel

"Pessimist is nothing but a well-informed optimist"




                                                                                
                   
                    "Nathan M. Andelin"                                         
                   
                    <nathanma@haaga.com       To:     <MIDRANGE-L@midrange.com> 
                   
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                    Sent by:                  Subject:     Re: how is system 
geared down?          
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                    05/10/2001 07:39 PM                                         
                   
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I'll offer another hypothesis.  This one is based on a little more on
research.  But admitedly, only a theory.

One component of IBM systems is known as the "system clock".  It provides a
signal that defines a base Mhz frequency used by other components,
including
the CPU.  Actually, RAM, cache, bus, and I/O processor speed may all be
synchronized with it.

The PowerPC and other components use a circuit known as a "phase-lock-loop"
(PLL) which synchronizes the various components and subsystems with the
system clock.  The CPU has a maximum recommended MHZ frequency, but can
actually be configured to run at some multiple of the system clock.  The
ratio between system clock and CPU speed can range from one-to-one (1:1) up
through ten-to-one (10:1).  Some processors don't support that wide of a
range.  For example, a processor may have a maximum recommended speed of
200
Mhz and support PLL ratios of 1:1 - 4:1.  If the system clock is set at 50
Mhz, and the PLL ratio is set at 1:1, then the CPU runs at 50 Mhz.  If the
system clock is set at 50 Mhz and the PLL ratio is configured at 4:1, then
the CPU runs at 200 Mhz.

Most of this information is a synopsis of my understanding of documents
that
can be found at www.chips.ibm.com, but you can also go to www.ibm.com and
do
a search on SYSCLK, PLL, system clock, etc.

My theory is that Rochester uses the PPL configuration ratio to "gear down"
the CPU from its maximum recommended Mhz rating.

Nathan.


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